Food Ingredient Structure and Function
Laboratory practical notes

Characteristics of lipids: Measurement of rancidity and peroxide value
Aims
• To measure the peroxide value of a series of oil samples;
• To obtain the most accurate and precise results possible with this method by selecting and applying the most appropriate laboratory equipment, glassware and practices;
• To consider selection of an appropriate blank determination;
• To draw conclusions on the factors influencing the reliability of the results;
• To assess the precision and accuracy of the procedure along with the factors impacting on the results

Background on peroxide value

There are a variety of approaches available allowing measurement of the extent to which rancidity reactions have occurred during the storage of a fat or oil. The most widely used is the analysis of the peroxide value of the oil.

The double bonds found in fats and oils play a role in autoxidation. Oils with a high degree of unsaturation are most susceptible to autoxidation. The best test for autoxidation (oxidative rancidity) is determination of the peroxide value. Peroxides are intermediates in the autoxidation reaction.

Autoxidation is a free radical reaction involving oxygen that leads to deterioration of fats and oils which form off-flavours and off-odours. Peroxide value, concentration of peroxide in an oil or fat, is useful for assessing the extent to which spoilage has advanced.

The peroxide value is defined as the amount of peroxide oxygen per 1 kg of fat or oil. Traditionally this was expressed in units of milliequivalents, although if we are using SI units then the appropriate option would be in millimoles per kg (NB. 1 millimole = 2 milliequivalents). Note also that the unit of milliequivalent has been commonly abbreviated as mequiv or even as meq.

The peroxide value is determined by measuring the amount of iodine which is formed by the reaction of peroxides (formed in fat or oil) with iodide ion.

2 I- + H2O + ROOH ---> ROH + 20H- + I2

Note that the base produced in this reaction is taken up by the excess of acetic acid present. The iodine liberated is titrated with sodium thiosulphate.

2S2O32- + I2 ---> S4O62- + 2 I-

The acidic conditions (excess acetic acid) prevents formation of hypoiodite, which would interfere with the reaction.

The indicator used in this reaction is a starch solution where amylose forms a blue to black solution with iodine and is colourless where iodine is titrated.

A precaution that should be observed is to add the starch indicator solution only near the end point (the end point is near when fading of the yellowish iodine colour occurs) because at high iodine concentration starch is decomposed to products whose indicator properties are not entirely reversible.

Correlation of rancid taste and peroxide value depends on the type of oil and is best tested with taste panels. The odours and flavours associated with typical oxidative rancidity are mostly due to carbonyl-type compounds. The shorter-chain aldehydes and ketones isolated from rancid fats are due to oxidative fission and are associated with advanced stages of oxidation. The carbonyl-type compounds develop in low concentrations early in the oxidative process.

Peroxide values of fresh oils are less than 10 milliequivalents /kg, when the peroxide value is between 20 and 40 milliequivalents/kg, a rancid taste is noticeable.

Reagents
Glacial acetic acid
Chloroform
Potassium iodide (saturated) solution
0.01 M standardised sodium thiosulphate
1% starch solution (used as an indicator)

Apparatus
Conical flasks (stoppered type)
Pipette (of suitable accuracy)
Burette

Procedure

1. Before commencing analyses be sure to record the following information: the actual concentration of the thiosulphate solution. You are told that this is nominally 0.01M. However, the concentration has been accurately determined and the results are recorded on the bottle. You will need these for your calculations (this is the y value in the equation provided).

2. Also record any relevant information from the oil container and its label. The factors influencing the development of rancidity are time and double bonds. Hence Use-by, best-before and fat compositional data may be useful for your report.

3. Preliminary measurement of peroxide value of you sample. Carry out a preliminary measurement of your sample. This requires that you try a sample weight somewhere in the range of 1 to 4 grams of the oil. Accurately weigh about 1 to 4 g. You will require a smaller sample if the oil is expected to be rancid or a larger sample for a relatively fresh oil sample. Select an appropriate balance that will give you a weighing that in turn will provide a reliable peroxide value. Weigh the oil or fat into a clean dry 250 mL stoppered conical flask.

4. Add 10 mL chloroform, using an operating fume cupboard. How accurately do we need to measure this volume? What type of glassware is appropriate?

5. Dissolve the fat by swirling, add 15 mL of glacial acetic acid, and 1 mL of fresh saturated potassium iodide solution. In both cases select a suitable piece of glassware for the purpose.

6. Stopper the flask, shake for 1 minute, allow to stand for 1 minute in the dark.

7. Slowly titrate (V mL) the freed iodine with standardised 0.01 M sodium thiosulphate solution, using 0.5 mL 1% starch solution as an indicator. The endpoint is reached when the colour just disappears. Note that the colour may be a dark blue-purple colour or, in the case of less rancid samples, may be a yellow brown colour. The end-point occurs where the last traces of the colour disappear. At the end-point cloudiness may remain.

8. Consider the preliminary titration value and decide what sample weight would be appropriate for subsequent measurements.

9. Carry out five final analyses (or more if the results are not concordant). Each time recording the actual weight of sample and the titration figure as accurately as possible.

10. Consider the selection of an appropriate blank determination. What is the purpose of the blank? What should you include in the blank?

11. Carry out five blank readings (or more if the results are not concordant).

12. Record all of your values (weights of samples, titres for blanks and samples) on the form attached. Show your data to your demonstrator before you do your clean up procedure.

Calculation

Peroxide value = (V - V₀) y x 10³ / actual weight of sample(g)

Where:
y = exact molarity of the sodium thiosulphate solution (as recorded on the bottle)
V = the individual titration figure for a sample (mL)
Vo = the mean of the replicate blank titration figures (mL)
Note that the units of peroxide values are milliequivalents per kg.

Report

Complete and submit the report form following the guidelines for preparing reports.

Food Ingredient Structure and Function Laboratory practical note...